U.S. patent number 9,476,374 [Application Number 14/520,237] was granted by the patent office on 2016-10-25 for method and device for reducing the emission of nitrous oxide.
This patent grant is currently assigned to ROBERT BOSCH GMBH. The grantee listed for this patent is Robert Bosch GmbH. Invention is credited to Alexandra Fuchsbauer, Uwe Mueller, Guido Porten, Matthias Walz.
United States Patent |
9,476,374 |
Fuchsbauer , et al. |
October 25, 2016 |
Method and device for reducing the emission of nitrous oxide
Abstract
In a method for regeneration of nitrogen oxide storage catalytic
converters in the exhaust gas tract of a gasoline engine assigned
an exhaust aftertreatment system in the form of a Y system
including two three-way catalytic converters, two downstream
nitrogen oxide storage catalytic converters, one shared catalytic
converter for selective catalytic reduction, and a rear nitrogen
oxide storage catalytic converter, a first part of the exhaust gas
in the first exhaust bank is set to rich and a second part of the
exhaust gas in the second exhaust bank is set to lean in a first
regeneration phase and subsequently, the second part of the exhaust
gas in the second exhaust bank is set to rich and the first part of
the exhaust gas in the first exhaust bank is set to lean in a
second regeneration phase.
Inventors: |
Fuchsbauer; Alexandra
(Stuttgart, DE), Mueller; Uwe (Cleebronn,
DE), Walz; Matthias (Wiernsheim, DE),
Porten; Guido (Wiernsheim, DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Robert Bosch GmbH |
Stuttgart |
N/A |
DE |
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Assignee: |
ROBERT BOSCH GMBH (Stuttgart,
DE)
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Family
ID: |
52824948 |
Appl.
No.: |
14/520,237 |
Filed: |
October 21, 2014 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20150107229 A1 |
Apr 23, 2015 |
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Foreign Application Priority Data
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Oct 22, 2013 [DE] |
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10 2013 221 421 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F01N
13/0097 (20140603); F01N 13/009 (20140601); F02D
41/0275 (20130101); F01N 13/011 (20140603); F02D
41/0082 (20130101); F01N 3/2073 (20130101); F01N
3/0871 (20130101); F01N 3/0842 (20130101); F01N
3/101 (20130101); F01N 3/2066 (20130101); F01N
2610/02 (20130101); Y02T 10/22 (20130101); Y02T
10/24 (20130101); Y02T 10/12 (20130101) |
Current International
Class: |
F01N
3/08 (20060101); F02D 41/02 (20060101); F01N
3/20 (20060101); F02D 41/00 (20060101); F01N
13/00 (20100101); F01N 3/10 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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103 93 184 |
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Sep 2005 |
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DE |
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10 2010 014 468 |
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Oct 2011 |
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DE |
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Primary Examiner: Bradley; Audrey K
Attorney, Agent or Firm: Norton Rose Fulbright US LLP
Claims
What is claimed is:
1. A method for regenerating nitrogen oxide storage catalytic
converters in an exhaust gas tract of a gasoline engine, the method
comprising: operating the gasoline engine predominantly lean;
supplying a first part of exhaust gas of the gasoline engine in a
first exhaust bank to a first three-way catalytic converter and to
a first nitrogen oxide storage catalytic converter connected
downstream; supplying a second part of the exhaust gas of the
gasoline engine in a second exhaust bank to a second three-way
catalytic converter and to a second nitrogen oxide storage
catalytic converter connected downstream, wherein the exhaust gas
is subsequently merged in a shared exhaust gas tract and supplied
to a further catalytic converter for selective catalytic reduction
and to a third nitrogen oxide storage catalytic converter connected
downstream; operating the first exhaust bank rich during the
regeneration, wherein the third nitrogen oxide storage catalytic
converter and the first nitrogen oxide storage catalytic converter
are regenerated during this time; operating the second exhaust bank
slightly lean so that a nitrogen oxide mass flow is generated in
the second bank, wherein in the catalytic converter for selective
catalytic reduction, ammonia of the first exhaust bank reacts with
nitrogen oxide of the second exhaust bank, so that nitrous oxide
cannot form at the third nitrogen oxide storage catalytic
converter; operating slightly lean the first exhaust bank, after
the regeneration of the first nitrogen oxide storage catalytic
converter and at least a partial regeneration of the third nitrogen
oxide storage catalytic converter, wherein a residual quantity of
ammonia is stored in the catalytic converter for selective
catalytic reduction from the rich operation of the first exhaust
bank for reacting with the nitrogen oxide; and operating the second
exhaust bank, which has previously been operated lean, using a rich
exhaust gas, so that the second nitrogen oxide storage catalytic
converter and the third nitrogen oxide storage catalytic converter
are regenerated; wherein the regeneration is completed when all of
the nitrogen oxide storage catalytic converters are
regenerated.
2. The method as recited in claim 1, wherein during the
regeneration, the exhaust gas compositions in the first exhaust
bank and in the second exhaust bank are selected to achieve a rich
exhaust gas mixture present in the shared exhaust gas tract.
3. The method as recited in claim 1, wherein the sequential
regeneration of the first nitrogen oxide storage catalytic
converter and the second nitrogen oxide storage catalytic converter
takes place only above a predefined first temperature of the
catalytic converter for selective catalytic reduction.
4. The method as recited in claim 1, wherein at a temperature of
the catalytic converter for selective catalytic reduction which is
below a predefined second temperature for the regeneration of the
first, second and third nitrogen oxide storage catalytic
converters, the first part of the exhaust gas in the first exhaust
bank and the second part of the exhaust gas in the second exhaust
bank are set to rich.
5. The method as recited in claim 4, wherein the gasoline engine is
operated lean for a predefined time period immediately after the
regeneration of the first, second and third nitrogen oxide storage
catalytic converters.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for reducing the emission
of nitrous oxide during or after a regeneration of nitrogen oxide
storage catalytic converters in the exhaust gas tract of a gasoline
engine which is operated predominantly lean, a first part of the
exhaust gas of the gasoline engine being supplied to a first
three-way catalytic converter in a first exhaust bank and to a
first nitrogen oxide storage catalytic converter connected
downstream, a second part of the exhaust gas of the gasoline engine
being supplied to a second three-way catalytic converter in a
second exhaust bank and to a second nitrogen oxide storage
catalytic converter connected downstream, and the exhaust gas being
subsequently merged in a shared exhaust gas tract and being
supplied to a catalytic converter for selective catalytic reduction
(SCR) and to a third nitrogen oxide storage catalytic converter
connected downstream. The present invention further relates to a
corresponding device for carrying out the method according to the
present invention.
2. Description of the Related Art
For lean operated gasoline engines it is known to provide an
exhaust aftertreatment using a three-way catalytic converter and a
downstream nitrogen oxide storage catalytic converter. Exhaust
emission control takes place in the stochiometric operation of the
gasoline engine with the aid of the three-way catalytic converter.
The nitrogen oxide storage catalytic converter, also referred to as
a nitrogen oxide storage/reduction catalytic converter or NSC,
stores the nitrogen oxides which arise during lean operation.
Nitrogen oxide storage catalytic converters work discontinuously in
a mode which includes two phases: In the first, longer phase, the
so-called lean phase (lambda>1), the nitrogen oxides of the
engine which are contained in the exhaust gas are stored. In the
second, shorter phase, the so-called rich phase (lambda<1), the
stored nitrogen oxides are regenerated with the aid of rich exhaust
gas which is generated within the engine. During the regeneration,
only nitrogen (N.sub.2), water (H.sub.2O) and carbon dioxide
(CO.sub.2) are formed from the stored nitrogen oxides in the case
of the usual operating mode of an NSC.
During the regeneration phase (lambda<1) of the nitrogen oxide
storage catalytic converter, ammonia (NH.sub.3) may be formed there
in secondary reactions under certain operating conditions of the
engine, such as at a low temperature of the three-way catalytic
converter. The ammonia causes nitrous oxide (laughing gas,
N.sub.2O) to be formed in the nitrogen oxide storage catalytic
converter. Nitrous oxide has a very high global warming potential.
For this reason, the emission of nitrous oxide is subject to a
strict limiting value, for example in the USA.
A known possibility of reducing the emission of nitrous oxide is
the additional use of a catalytic converter for selective catalytic
reduction (SCR catalytic converter) upstream from the nitrogen
oxide storage catalytic converter. The ammonia generated in the
three-way catalytic converter is stored in the SCR catalytic
converter and converted with the arising NO.sub.x or directly with
the existing NO.sub.x during lean operation.
A method is known from the published German patent application
document DE102010014468A1 for reducing harmful exhaust gases of a
lean operated internal combustion engine by using an exhaust
aftertreatment system including a first NO.sub.x storage catalytic
converter which is positioned upstream and followed by an N.sub.2O
reduction catalytic converter, including the steps: a) conveying a
lean exhaust gas through the NO.sub.x storage catalytic converter
during normal operation; b) supplying an exhaust gas having
.lamda..ltoreq.1 to the N.sub.2O reduction catalytic converter
shortly before or simultaneously with the initiation of step c); c)
conveying an exhaust gas mixture having .lamda..ltoreq.1 through
the NO.sub.x storage catalytic converter until the latter is
sufficiently regenerated; d) discontinuing normal operation.
In the illustrated exemplary embodiments, the exhaust gas is
conveyed, in this case, in a single-flow exhaust gas tract through
a three-way catalytic converter, an NO.sub.x storage catalytic
converter as well as a final N.sub.2O reduction catalytic
converter. The N.sub.2O reduction catalytic converter may be
designed as a three-way catalytic converter, as an NO.sub.x
reduction catalytic converter, as an NO.sub.x storage catalytic
converter, or as an oxidation catalytic converter. According to the
present invention, rich exhaust gas is conveyed past the NO.sub.x
storage catalytic converter to the N.sub.2O reduction catalytic
converter simultaneously or shortly before the regeneration of the
NO.sub.x storage catalytic converter. In this way, a sufficient
amount of reduction agent at the N.sub.2O reduction catalytic
converter is available for the purpose of reducing the occurring
nitrous oxide during the regeneration of the NO.sub.x storage
catalytic converter.
In contrast thereto, exhaust systems are known which are
constructed in a so-called Y system. The exhaust system is, in this
case, implemented in two exhaust gas tracts (banks), which are
close to the engine and which are assigned to two cylinder groups
of the internal combustion engine, and a downstream, joint exhaust
gas tract. A three-way catalytic converter (TWC) and a nitrogen
oxide storage catalytic converter may be situated in each of the
exhaust gas tracts which are close to the engine, and an SCR
catalytic converter for selective catalytic reduction of NO.sub.x
using ammonia as the reduction agent as well as a downstream
nitrogen oxide storage catalytic converter may be situated in the
joint exhaust gas tract.
In such a Y system, ammonia is formed at the two three-way
catalytic converters during the regeneration of the nitrogen oxide
storage catalytic converters using a rich exhaust gas mixture. At
the two front nitrogen oxide storage catalytic converters, no or
only very little nitrous oxide is formed therefrom since they are
relatively hot during operation. The ammonia is stored in the SCR
catalytic converter, whereby no or only little nitrous oxide is
generated even in a cold, rear nitrogen oxide storage catalytic
converter. If, however, another lean operation does not take place
in such a system after the regeneration of the nitrogen oxide
storage catalytic converter, ammonia remains stored at low SCR
temperatures and is then discharged at higher SCR temperatures.
This ammonia reacts in the downstream nitrogen oxide storage
catalytic converter to become nitrous oxide. It is possible in
gasoline engines, in particular, that the regeneration is followed
by a regular operating phase in the case of a stochiometric or rich
air/fuel mixture, thus resulting in the above-described formation
of nitrous oxide.
The published German patent application document DE10393184T5
describes a system for treating exhaust gases which are emitted by
a vehicle, including: a) a multi-cylinder diesel engine including a
first exhaust manifold in flow connection with a first plurality of
cylinders and a second exhaust manifold in flow connection with a
deviating, second plurality of cylinders, b) a first NO.sub.x
absorption catalytic converter in a first exhaust branch in flow
connection with the first manifold, c) a second NO.sub.x absorption
catalytic converter in a second exhaust branch in flow connection
with the second manifold, d) a shared exhaust branch having an
inlet in flow connection with the first and the second exhaust
branches downstream from the first and the second NO.sub.x
catalytic converter, an oxidation catalytic converter being
situated in the shared exhaust branch through which exhaust gases
flow from the first and the second branches after being merged in
the shared exhaust branch, and e) an ECU means which controls the
composition of exhaust gases in the first exhaust manifold
independently of the composition of the exhaust gases in the second
exhaust manifold according to a programmed routine for the purpose
of periodically generating rich gases in the one exhaust manifold
and lean gases in the other exhaust manifold.
The publication thus provides a system and a method for
regenerating nitrogen oxide storage catalytic converters in
multi-cylinder diesel engines having a Y exhaust system including
two exhaust manifolds, in each of which a nitrogen oxide storage
catalytic converter is provided. The exhaust manifolds are followed
by a shared exhaust branch including an oxidation catalytic
converter. To regenerate the nitrogen oxide storage catalytic
converters, in a first step, a first cylinder bank is operated rich
and a second cylinder bank is operated lean so that in one exhaust
manifold, a rich exhaust gas is present and in the second exhaust
manifold, a lean exhaust gas is present. The nitrogen oxide storage
catalytic converter which is exposed to the rich exhaust gas is
regenerated while NO.sub.x from the lean exhaust gas is absorbed by
the second nitrogen oxide storage catalytic converter. In a next
step, the first cylinder bank is operated lean and the second
cylinder bank is operated rich, and the second nitrogen storage
catalytic converter is regenerated. The control of the exhaust gas
composition takes place in both steps in such a way that the joint
exhaust gas flow remains lean so that the oxidation catalytic
converter is able to oxidize the excessive reduction agent (HC, CO)
and thus prevent a reduction agent slip.
According to the description, the method and the system may also be
used for gasoline-operated lean engines.
The method thus allows for the emission of HC and CO to be reduced
during the regeneration of nitrogen oxide storage catalytic
converters in a Y exhaust system. What is not described is the
prevention of the emission of nitrous oxide in a Y exhaust system
which is formed at a nitrogen oxide storage catalytic converter
from the ammonia which is generated at a three-way catalytic
converter, which is connected upstream, during the regeneration
phase of nitrogen oxide storage catalytic converters.
It is thus the object of the present invention to provide a method,
with the aid of which the emission of nitrous oxide may be reduced
in gasoline engines which are predominantly operated lean and
include a Y exhaust system.
A further object of the present invention is to provide a
corresponding device for carrying out the method.
BRIEF SUMMARY OF THE INVENTION
The object of the present invention which is related to the method
is achieved in that for the regeneration of nitrogen oxide storage
catalytic converters, the first part of the exhaust gas in the
first exhaust bank is set to rich and the second part of the
exhaust gas in the second exhaust bank is set to lean in a first
regeneration phase until the first nitrogen oxide storage catalytic
converter is regenerated, and in that subsequently, the second part
of the exhaust gas in the second exhaust bank is set to rich and
the first part of the exhaust gas in the first exhaust bank is set
to lean in a second regeneration phase until the second nitrogen
oxide storage catalytic converter and the third nitrogen oxide
storage catalytic converter are regenerated.
During the first regeneration phase, the first nitrogen oxide
storage catalytic converter is regenerated entirely and the third,
shared nitrogen oxide storage catalytic converter is regenerated at
least partially. Ammonia is formed at the first three-way catalytic
converter in the process. At the first nitrogen oxide storage
catalytic converter which directly connects to the first three-way
catalytic converter, there is no or only very little formation of
nitrous oxide due to the relatively high temperatures. The second
exhaust bank is operated slightly lean, and therefore, an NO.sub.x
mass flow is generated on the bank. In the following catalytic
converter for selective catalytic reduction, the ammonia from the
first exhaust bank reacts with the nitrogen oxide from the second
exhaust bank. Consequently, no ammonia reaches the third nitrogen
oxide storage catalytic converter, whereby the emission of nitrous
oxide is largely prevented. The exhaust gas composition in the
first exhaust bank must be sufficiently rich in this case in order
to also enable a regeneration of the third nitrogen oxide storage
catalytic converter and to prevent a nitrogen oxide slip.
After the regeneration of the first nitrogen oxide storage
catalytic converter and an at least partial regeneration of the
third nitrogen oxide storage catalytic converter during the first
regeneration phase, the first exhaust bank is initially set to lean
and then the second exhaust bank is set to rich to carry out the
second regeneration phase. During the second regeneration phase,
the regeneration of the second nitrogen oxide storage catalytic
converter and the final regeneration of the third nitrogen oxide
storage catalytic converter take place analogously to the first
regeneration phase. The second regeneration is completed when all
three nitrogen oxide storage catalytic converters are
regenerated.
The sequential procedure may largely prevent the emission of
nitrous oxide in the above-described Y exhaust system.
For this purpose, it may be provided that during the first
regeneration phase, the compositions of the first part of the
exhaust gas and of the second part of the exhaust gas are selected
in such a way that after the completion of the first regeneration
phase, at least a residual quantity of ammonia remains stored in
the catalytic converter for selective catalytic reduction. During
the adjustment from the first regeneration phase to the second
regeneration phase, the exhaust bank which was regenerated first is
preferably initially set to slightly lean. The nitrogen oxide which
is now available reacts with the ammonia which is stored in the
catalytic converter for the selective catalytic reduction.
In order to ensure the regeneration of the third, shared nitrogen
oxide storage catalytic converter, it may be provided that during
the first regeneration phase and the second regeneration phase, the
exhaust gas compositions in the first exhaust bank and in the
second exhaust bank are selected in such a way that a rich exhaust
gas mixture is present in the shared exhaust gas tract.
The method may be applied particularly advantageously in the case
of a comparably small storage capacity of the catalytic converter
for selective catalytic reduction for ammonia, as is the case at
high temperatures of the catalytic converter for selective
catalytic reduction. It may therefore be provided that the
sequential regeneration of the first nitrogen oxide storage
catalytic converter and the second nitrogen oxide storage catalytic
converter takes place only above a predefined first temperature of
the catalytic converter for selective catalytic reduction.
According to one preferred embodiment variant of the present
invention, it may be provided that at a temperature of the
catalytic converter for selective catalytic reduction which is
below a predefined second temperature for regeneration of the
nitrogen oxide storage catalytic converters, the first part of the
exhaust gas in the first exhaust bank and the second part of the
exhaust gas in the second exhaust bank are set to rich and that the
gasoline engine is imperatively operated lean for a predefined time
period immediately after the regeneration of the nitrogen oxide
storage catalytic converters.
The second temperature may in this case correspond to the
above-mentioned first temperature.
During the regeneration of the nitrogen oxide storage catalytic
converters, a rich exhaust gas having a lambda<1 is set in both
exhaust banks, whereby all three nitrogen oxide storage catalytic
converters are regenerated at the same time. During the
regeneration phase, ammonia is formed at the two three-way
catalytic converters. At the nitrogen oxide storage catalytic
converters which are directly connected to the three-way catalytic
converters, no or only a small amount of nitrous oxide is formed
due to the high temperatures prevailing there. In the downstream
catalytic converter for selective catalytic reduction, the ammonia
which is formed at the three-way catalytic converters is stored so
that no ammonia is present at the third nitrogen oxide storage
catalytic converter and thus no nitrous oxide is formed. The
regeneration phase is continued until all three nitrogen oxide
storage catalytic converters are regenerated.
In the lean phase which now follows and is imperatively predefined
according to the present invention, a part of the nitrogen oxides
which are generated in the process reacts with the ammonia which is
stored in the catalytic converter for selective catalytic
reduction. The ammonia is thus not discharged due to a temperature
increase of the catalytic converter for selective catalytic
reduction, for example, during a rich operating phase of the
gasoline engine which follows the regeneration and is conveyed to
the third nitrogen oxide storage catalytic converter, whereby the
formation of nitrous oxide is prevented.
The method variant is particularly advantageous at comparably low
temperatures of the catalytic converter for selective catalytic
reduction and the high ammonia storage capacity associated with it
and thus complements the previously described, sequential
regeneration of the nitrogen oxide storage catalytic
converters.
The object of the present invention which relates to the device is
achieved in that a circuit or a program sequence is provided in the
control unit for setting a rich exhaust gas in the first exhaust
bank and a lean exhaust gas in the second exhaust bank during a
first regeneration phase of the nitrogen oxide storage catalytic
converters and for setting a lean exhaust gas in the first exhaust
bank and a rich exhaust gas in the second exhaust bank during a
second regeneration phase of the nitrogen oxide storage catalytic
converters. The device thus allows for the described method to be
carried out.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic representation of a gasoline engine
including a single-flow exhaust aftertreatment system.
FIG. 2 shows a schematic representation of a gasoline engine
including an exhaust aftertreatment system in the design of a Y
system.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a schematic representation of a gasoline engine 10
which is operated predominantly lean and includes a single-flow
exhaust aftertreatment system 20. In this case, the exhaust gas of
gasoline engine 10 is initially supplied in an exhaust gas tract 21
to a three-way catalytic converter 22 and subsequently, to a
nitrogen oxide storage catalytic converter 23.
The exemplary embodiment shows the sequence in principle for the
formation of nitrous oxide using the example of a known,
single-flow exhaust system.
Three-way catalytic converter 22 is used for exhaust emission
control in the stochiometric operation of gasoline engine 10.
Nitrogen oxides which are formed during the lean operation of
gasoline engine 10 are stored in nitrogen oxide storage catalytic
converter 23. When the storage capacity of nitrogen oxide storage
catalytic converter 23 is exhausted, the latter is regenerated
through an operation of gasoline engine 10 using a rich air/fuel
mixture at a lambda value of <1.
During the regeneration of nitrogen oxide storage catalytic
converter 23 at lambda<1, ammonia (NH.sub.3) may be formed in
secondary reactions at three-way catalytic converter 22 under
certain conditions of the engine such as at a comparably low
temperature of three way catalytic converter 22. The ammonia causes
nitrous oxide (laughing gas, N.sub.2O) to be formed in downstream
nitrogen oxide storage catalytic converter 23. Nitrous oxide has a
very high global warming potential, which is why its emission is
subject to a strict limiting value, for example in the USA.
For diesel engines, it is known to provide a catalytic converter
for selective catalytic reduction (SCR catalytic converter), which
is not illustrated here, between three-way catalytic converter 22
and nitrogen oxide storage catalytic converter 23. The SCR
catalytic converter is used to reduce the nitrogen oxide emissions
which arise during the provided, lean operation of diesel engines.
For this purpose, ammonia is formed from an aqueous urea solution,
which is supplied to the exhaust gas, and then converted into
nitrogen and water in the SCR catalytic converter using the
nitrogen oxides which are contained in the exhaust gas. The SCR
catalytic converter thus also prevents the ammonia which is formed
at three-way catalytic converter 22 from reaching nitrogen oxide
storage catalytic converter 23 and from being converted there into
nitrous oxide.
FIG. 2 shows a schematic representation of a gasoline engine 10
including an exhaust aftertreatment system 30 in the design of a Y
system.
Exhaust aftertreatment system 30 is implemented as a Y system in
two exhaust banks 31, 34, which are close to the engine and which
are assigned to two cylinder groups of gasoline engine 10, and a
downstream, shared exhaust gas tract 37. In first exhaust bank 31,
a first three-way catalytic converter 32 is situated and directly
followed by a first nitrogen oxide storage catalytic converter 33.
In second exhaust bank 34, a second three-way catalytic converter
35 is situated and directly followed by a second nitrogen oxide
storage catalytic converter 36. In shared exhaust gas tract 37, a
catalytic converter 38 for selective catalytic reduction of
nitrogen oxides using ammonia as the reduction agent is provided as
well as a downstream third nitrogen oxide storage catalytic
converter 39 as the main nitrogen oxide catalytic converter.
In such a Y system, ammonia is formed at the two three-way
catalytic converters 32, 35 during the regeneration of nitrogen
oxide storage catalytic converters 33, 36, and 39 having
lambda<1. At catalytic converter 38 for selective catalytic
reduction, the ammonia is broken down, whereby no or only a small
amount of nitrous oxide is generated in third nitrogen oxide
storage catalytic converter 39. For the ammonia to be converted in
nitrogen oxide storage catalytic converter 39, nitrogen oxides from
the exhaust gas are necessary, such as the ones generated during
lean operation of gasoline engine 10. If after the regeneration of
nitrogen oxide storage catalytic converters 33, 36, and 39 a lean
operation does not take place as is possible in gasoline engines as
a function of the operating situation, ammonia remains stored, in
particular, at a low temperature of catalytic converter 38 for
selective catalytic reduction. This ammonia is then discharged at
higher temperatures of catalytic converter 38 for selective
catalytic reduction and conveyed, together with the exhaust gas, to
third nitrogen oxide storage catalytic converter 39, where it is
converted into nitrous oxide.
It is therefore provided according to the present invention that
during the regeneration of nitrogen oxide storage catalytic
converters 33, 36, and 39, one exhaust bank 31, 34 is operated rich
and the other exhaust bank 31, 34 is operated lean at the same
time. For this purpose, first exhaust bank 31 is initially operated
rich during the regeneration, for example, and first nitrogen oxide
storage catalytic converter 33 and third nitrogen oxide storage
catalytic converter 39 are regenerated during this time. Ammonia is
formed at first three-way catalytic converter 32 in the process. At
first nitrogen oxide storage catalytic converter 33 which directly
follows first three-way catalytic converter 32, there is no or only
very little formation of nitrous oxide due to the relatively high
temperatures. Second exhaust bank 34 is operated slightly lean and
therefore, a nitrogen oxide mass flow is generated in second bank
34. In following catalytic converter 38 for selective catalytic
reduction, the ammonia of first exhaust bank 31 reacts with the
nitrogen oxide of second exhaust bank 34. Consequently, nitrous
oxide cannot form at third nitrogen oxide storage catalytic
converter 39. The exhaust gas which is conveyed in first exhaust
bank 31 must be sufficiently rich in order to also allow for a
regeneration of third nitrogen oxide storage catalytic converter 39
and to prevent a nitrogen oxide slip. After the regeneration of
first nitrogen oxide storage catalytic converter 33 and an at least
partial regeneration of third nitrogen oxide storage catalytic
converter 39, the exhaust gas of first exhaust bank 31 is initially
set to slightly lean. A residual quantity of ammonia is
advantageously still stored in catalytic converter 38 for selective
catalytic reduction from the rich phase of first exhaust bank 31
for the purpose of reacting with the now present nitrogen oxide.
Second exhaust bank 34 which has previously been operated lean is
now operated using a rich exhaust gas, and thus second nitrogen
oxide storage catalytic converter 36 and third nitrogen oxide
storage catalytic converter 39 are regenerated. In this case,
ammonia which does not react to nitrous oxide at the downstream,
second nitrogen oxide storage catalytic converter due to the
relatively high temperatures is formed at second three-way
catalytic converter 35. In catalytic converter 38 for selective
catalytic reduction, the ammonia reacts with the nitrogen oxide of
first exhaust bank 31 so that ammonia is now still not present at
third nitrogen oxide storage catalytic converter 39 for the
formation of nitrous oxide. The regeneration is completed when all
three nitrogen oxide storage catalytic converters 33, 36, and 39
are regenerated.
The method may be particularly advantageously used at a comparably
high temperature and an accordingly low ammonia storage capacity of
catalytic converter 38 for selective catalytic reduction.
In order to also effectively reduce or prevent the formation of
nitrous oxide at low temperatures and an accordingly high ammonia
storage capacity of catalytic converter 38 for selective catalytic
reduction, it is provided in one method variant to operate both
exhaust banks 31, 34 rich below a predefined temperature of
catalytic converter 38 for selective catalytic reduction for the
generation of nitrogen oxide storage catalytic converters 33, 36,
and 39 and to imperatively predefine a lean phase of gasoline
engine 10 after the regeneration of nitrogen oxide storage
catalytic converters 33, 36, and 39.
In this case, both exhaust banks 31, 34 are initially operated rich
for the regeneration of nitrogen oxide storage catalytic converters
33, 36, and 39. Ammonia is generated in the process at both
three-way catalytic converters 32, 35 in a secondary reaction. At
nitrogen oxide storage catalytic converters 33, 36 which directly
follow three-way catalytic converters 32, 35, no or only very
little formation of nitrous oxide takes place due to the relatively
high temperatures. In downstream catalytic converter 38 for
selective catalytic reduction, the ammonia which was formed at
three-way catalytic converters 32, 35 is stored so that no nitrous
oxide is initially formed at third nitrogen oxide storage catalytic
converter 39. The regeneration is finished after all three nitrogen
oxide storage catalytic converters 33, 36, and 39 have been
regenerated. In a following, imperatively predefined lean phase of
gasoline engine 10, a part of the nitrogen oxides, which are formed
in the process, is reduced at catalytic converter 38 for selective
catalytic reduction using the stored ammonia. Therefore, ammonia
cannot be discharged from catalytic converter 38 for selective
catalytic reduction and conveyed to third nitrogen oxide storage
catalytic converter 39 during further operation. The formation of
nitrous oxide may thus be largely prevented.
As a result of the sequential procedure during the regeneration of
nitrogen oxide storage catalytic converters 33, 36, and 39, the
emission of nitrous oxide may be considerably reduced. In this
case, the sequential regeneration of first nitrogen oxide storage
catalytic converter 33 in first exhaust gas tract 31 and of second
nitrogen oxide storage catalytic converter 36 in second exhaust gas
tract 34 is advantageously used at a high temperature and the
accordingly low ammonia storage capacity of catalytic converter 38
for selective catalytic reduction, while alternatively, the
simultaneous regeneration of nitrogen oxide storage catalytic
converters 33, 36, and 39 takes place at a low temperature and the
accordingly high ammonia storage capacity of catalytic converter 38
for selective catalytic reduction and is followed by a prescribed
lean phase of the gasoline engine.
* * * * *